Bone is a composite material composed of impure hydroxyapatite, collagen, and a variety of non-collagenous proteins, as well as embedded and adherent cells. Bone can be processed into an implantable biomaterial, such as an allograft, for example, by removing the cells, leaving behind the extracellular matrix. The processed bone biomaterial can have a variety of properties, depending upon the specific processes and treatments applied to it, and may incorporate characteristics of other biomaterials with which it is combined. For example, bone-derived biomaterials may be processed into load-bearing mineralized grafts that support and integrate with the patient's own bone or may alternatively be processed into soft, moldable, or flowable demineralized bone biomaterials that have the ability to induce a cellular healing response.
The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which the injured bone is unable to support physiologic loading. Metal pins, screws, and meshes are frequently needed to replace the mechanical functions of injured bone. However, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Furthermore, most metal implants are permanent and unable to participate in physiological remodeling.
Bone's cellular healing processes, through bone tissue formation by osteoblast cells coordinated with bone and graft resorption by osteoclast cells, permit bone grafts and certain bone substitute materials to remodel into endogenous bone that is almost indistinguishable from the original. However, the use of bone grafts is limited by the available shape and size of grafts and the desire to optimize both mechanical strength and degradation rate. Variations in bone size and shape among patients (and donors) also make bone grafts a less optimal substitute material. Bone substitute materials and bone chips are quickly remodeled but cannot immediately provide mechanical support. In contrast, cortical bone grafts can support physiological stresses but remodel slowly.
U.S. Pat. Nos. 5,899,939; 5,507,813; 6,123,731; 6,294,041; 6,294,187; 6,332,779; 6,440,444; and 6,478,825; the contents of all of which are incorporated herein by reference, describe methods for preparing composites including allogenic bone for use in load bearing orthopedic applications.
Thus, it is desirable to have a bone substitute material for structural grafts that may be produced in larger quantities than grafts derived solely from bone and that may be fabricated or molded into shapes without being limited by the shape of the originating tissue. It is also desirable to have injectable bone substitute materials that may be implanted using minimally invasive techniques.